Semiconductor integrated circuit (IC) technology has experienced rapid progress including the continued minimization of feature sizes and the maximization of packing density. The minimization of feature size relies on improvements in photolithography and its ability to print smaller features or critical dimensions (CD). Various resolution enhancement techniques (RET) have been developed to enhance the resolution of photolithography. An example of such a technique is off-axis illumination (OAI). OAI illumination may include any radiation that reduces and/or eliminates the “on-axis” (on the optical axis) component of the radiation. For OAI, however, the shape and size of the illumination must be designed and/or optimized for the specific pattern that is to be printed on a substrate. Another RET that is often used together with OAI is double dipole lithography (DDL) or double patterning. In DDL, patterns are split into a vertical pattern and a horizontal pattern, which in turn requires two (double) exposure processes. Each of the exposure processes in DDL may use an appropriate OAI. However, even with DDL and OAI, loading effects during mask fabrication are still a concern where a single mask (or reticle) may include dense patterns and isolated patterns.
Improvements in one or more of these areas are needed.